on-line-segment-x-y-p

Returns true if a point lies on (or nearly on) a line segment amd
returns nil otherwise.

The first two arguments are real numbers for the X and Y coordinates
of the point, and the remaining required arguments are real numbers
for the endpoints of the line segment.

The margin argument is a real number
indicating the distance from the line within which the point must lie
to be considered on the line. If it is within this distance, then true
is returned, and otherwise nil is returned.